Can you skip frames with no animation?
You can skip frames where nothing changes.

Probably the simplest, is to manually render out different sections of the animations. It works by splitting the different parts into sections, and you render out each of these sections individually.

You do this by setting the start and end frame to match the parts of the animation you want to render, and then pressing animation, making sure that you have it outputting to the correct folder. You then repeat the process. With the parts that aren't animated at all, just render by pressing the render button, or by setting the start and end frame settings to be the same frame (making them only render that frame).

You then need to import everything into a video editor and stretch out the frames that where rendered as a single image instead of an animation.